A narrow complex tachycardia with varying RP intervals: What is the mechanism?

Transition from two wide to a narrow QRS complex tachycardia: What is the mechanism of tachycardia and transition?

“…This suggests the presence of multiple fast/retrograde AV nodal pathways, 21 and these transitions could be related to the second slow pathway 22–24 . Therefore, the multiple forms of induced tachycardias with both RBBB and LBBB morphologies favor atypical AVNRT 6,17,18 …”

“…[22][23][24] Therefore, the multiple forms of induced tachycardias with both RBBB and LBBB morphologies favor atypical AVNRT. 6,17,18 It would be only speculative to make a concise correlation with anatomy since the AH change over a slow pathway might be both due to functional decremental conduction or perhaps more than one distinct anatomical slow pathway. In the presence of dual AV nodal pathways, the refractory period of the fast pathway is usually longer than that of the slow pathway.…”

“…[1][2][3][4][5] Since the atrial activation must follow the ventricular activation in AVRT, 14 an H-A-V sequence does not occur as a rule, but this is not always for an NFRT/NVRT. 11,[15][16][17] One possibility is that AT conducts over the slow pathway to initiate typical (slow-fast) AVNRT 18 ; however, the AT would not be expected to change with the shift to a longer PR interval. 19,20 In current tracings, the transition shows that change to a slow AV nodal pathway also resets the tachycardia.…”